For high-vision transliteration! broadcasting using the MUSE format, the band-compressed image signal is transmitted as the sample value analog signal at a transmission rate of 16.2 MHz. On the receiver side, the received sample value analog signal is resampled at 16.2 MHz and the MUSE format is regenerated.
FIG. 14 is a diagram illustrating the configuration of a conventional noise-reducing circuit used for the MUSE data. At input terminal 100, the resample value obtained by means of sampling the demodulated sample values at 16.2 MHz using an A/D converter (not shown in the figure), that is, the digital resample value of 16.2 MHz corresponding to the analog sample value is input as the input image signal Sin. This input image signal Sin is sent to one input terminal of subtractor 102, and it is also sent to one input terminal of subtractor 104. The image signal two frames before (one picture before in the MUSE format) is applied to other input terminal of subtractor 104 as delay image signal Sd from frame memory 106. At subtractor 104, delay image signal Sd is subtracted from input image signal Sin to obtain the difference. The difference value E output from subtractor 104 is input to coefficient multiplier 108 consisting of ROM (lookup table), where it is multiplied with a constant K. The corrected difference value KE output from coefficient multiplier 108 is input to the other input terminal of subtractor 102. At subtractor 102, corrected difference value KE is subtracted from input image signal Sin. The output signal of subtractor 102 is output as output image signal Sout, and it is also input to frame memory 106.
In this noise-reducing circuit, the following formula is established in subtractor 102: EQU Sout=Sin-KE (1)
In subtractor 104, the following formula is established: EQU E=Sin-Z.sup.-1 .multidot.Sout (2)
where, Z.sup.-1 represents the delay time of frame memory 106. It is 1/15 sec in the case of the MUSE format.
From formulas (1) and (2), the following formula of the input/output characteristics or transmission function between input image signal Sin and output image signal Sout is obtained: EQU Sout/Sin=(1-K)/(1-KZ.sup.-1) (3)
On the other hand, as the noise is without correlation, the following input/output characteristics or transmission function is established between noise Nin contained in input image signal Sin and noise Nout contained in output image signal Sout. EQU Nout/Nin={(1-K)/(1-K.sup.2)}.sup.1/2 ( 4)
Consequently, by means of this noise-reducing circuit, the ratio of the image signal to noise (S/N ratio) can be increased by a factor of (1+K/1-K).sup.1/2, that is, the noise level can be reduced.
However, when a moving image portion is contained in the image, there is a significant change in the present pixel with respect to that one image before in the vicinity of the edge of the moving image portion. Consequently, the difference value E between input image signal Sin and delay image signal Sout is large. In the noise-reducing circuit, appropriate control is performed to ensure that if difference value E exceeds a prescribed limit, the limiter function of coefficient multiplier 108 takes place, multiplication coefficient K becomes null, and virtually no corrected difference value KE is sent to subtractor 102, and input image signal Sin is output directly as output image signal Sout. That is, for the edge of the moving image portion with a large change 30 that the difference value E is larger than a prescribed upper limit value, no noise reduction is performed.
On the other hand, when the difference between the background and the moving image portion is small, the change in the color in the vicinity of the edge of the moving image portion is relatively small, and a relatively small difference value E can be obtained from subtractor 104. However, in the conventional noise reducing circuit, if difference value E is smaller than the upper limit, for both the static picture and the moving image, constant K is multiplied by the difference value at coefficient multiplier 108, and corrected difference value KE is sent to subtractor 102. As a result, in the vicinity of the moving image portion in the regenerated image, the image is overlapped at the corresponding position on the preceding image, causing blurring in time. For example, as shown in FIG. 15, when the ball 122 in image 120 moves in the direction indicated by the arrow, a trailing phenomenon 124 takes place, with a residual image dragging its tail in the reverse direction to the moving direction (backward direction). This is a problem.
In this way, in the conventional noise reducing circuit, the noise is reduced by exploiting the correlation property among the frames. Consequently, it is impossible to perform noise reducing processing for the edge of the moving image portion with a large change, and adverse effects, such as the trailing phenomenon, etc., are caused for the edge of the moving image portion with a small change.
It is an object of this invention to provide a noise reducing circuit which performs the noise reduction effectively free of the trailing phenomenon and other adverse effects for the edge of a moving image portion.